1. Field of the Invention
The present invention relates to an output stage scheme, and more particularly to a hybrid output stage apparatus and related method thereof.
2. Description of the Prior Art
Normally, an operational amplifier is a two-stage configuration, which includes a first amplifying circuit (i.e. amplifying stage) and a second outputting circuit (i.e. output stage). Operational amplifiers can be further classified into class A amplifiers, class B amplifiers, and class AB amplifiers. Please refer to FIG. 1. FIG. 1 is a diagram illustrating a prior art class A amplifier 10, class B amplifier 20, and class AB amplifier 30, and their respective operational characteristics (i.e. the relationship between the output voltage and the driving current). According to FIG. 1(a), the P-type transistor Mp of the class A amplifier is conducting for the whole wave swing of the input signal Vin (the current Imp is the conduct current of the P-type transistor Mp), i.e. the conduct angle is 360°. Accordingly, the power efficiency of the class A amplifier 10 is not higher than 25%. According to FIG. 1(b), the bias current Ibias of the class A amplifier 10 should be relatively high so that the class A amplifier 10 have the conduct angle of 360°, and this is what causes the low power efficiency of the class A amplifier 10. Therefore, for some applications that require higher power efficiency, the class B amplifier 20 or the class AB amplifier 30 is a preferred choice. According to FIG. 1(c), when the class B amplifier 20 is in a static condition, the P-type transistor Mp and the N-type transistor Mn are just in the edge of the cut-off region. Therefore, both the P-type transistor Mp and the N-type transistor Mn respond to a half-wave swing of the input signal Vin (the current Imp is the conduct current of the P-type transistor Mp and the current Imn is the conduct current of the N-type transistor MN), i.e. a conduct angle of 180°. Accordingly, the power efficiency of the class B amplifier 20 is not higher than 78.5%. Furthermore, according to FIG. 1(d), to obtain the conduct angle of 180°, the bias current Ibias of the class B amplifier 20 should be equal to zero, so that the class B amplifier 20 has a higher power efficiency than the class A amplifier 10. However, because the static bias current Ibias is zero, it results in the class B amplifier 20 being turned off, and the output voltage Vo is much more easily interfered with by noise, that is to say, the distortion of the output voltage Vo is also more serious.
According to FIG. 1(e), the P-type transistor Mp and the N-type transistor Mn of the class AB amplifier 30 in the static condition are turned on slightly. Therefore, both the P-type transistor Mp and the N-type transistor Mn respond to at least a half-wave swing of the input signal Vin (the current Imp is the conduct current of the P-type transistor Mp and the current Imn is the conduct current of the N-type transistor MN), i.e. a conduct angle larger than 180°. Accordingly, the power efficiency of the class AB amplifier 30 is between the power efficiency of the class A amplifier 10 and the class B amplifier 20. Furthermore, according to FIG. 1(f), because the bias current Ibias of the class AB amplifier 30 is not zero in the static condition, the larger sized P-type transistor Mp and N-type transistor Mn will consume more static current of the class AB amplifier 30.
Please refer to FIG. 2. FIG. 2 is a diagram illustrating the bias voltage of a prior art class AB amplifier 200. The bias configuration of the class AB amplifier 200 is implemented by utilizing a current Io to flow through a resistor 202 (the value of the impedance is Z) formed by a transistor network. By selecting an appropriate value of Io×Z, the currents I(Mop1), I(Mon1) of the P-type transistor Mop1 and the N-type transistor Mon1 respectively will not be zero at the same time regardless of any value of the output voltage Vout. In other words, the class AB amplifier 200 has a better ability to resist noise. On the other hand, through an appropriate setting of the aspect ratio (W/L)2 of the N-type transistor Mon1, the class AB amplifier 200 will have the largest output current IN(max) as shown below:
                                                                        I                                  N                  ⁡                                      (                    max                    )                                                              =                            ⁢                              0.5                ×                                  K                  n                                ×                                                      (                                          W                      /                      L                                        )                                    2                                ×                                                      (                                          V                                              n                        ⁢                                                                                                  ⁢                        1                                                              )                                    2                                                                                                                        =                                ⁢                                  0.5                  ×                                      K                    n                                    ×                  X                  ×                                                            (                                              V                                                  n                          ⁢                                                                                                          ⁢                          1                                                                    )                                        2                                                              ,                                                where                  ⁢                                                                          ⁢                  X                                =                                                                            (                                              W                        /                        L                                            )                                        2                                    .                                                                                        (        1        )            
According to equation (1), Kn is the conductivity parameter of the N-type transistor. Therefore, static current still exists in the class AB amplifier 200. That is to say, the class AB amplifier 200 still has static power consumption problem in the static condition.